Say "Hello!" On Every Button Press

This tutorial will walk you through calling printf() in response to a button press.

Start a new application. A Tock application in C looks like a typical C application. Lets start with the basics:
```
#include <stdio.h>

int main(void) {
  return 0;
}
```
You also need a makefile. Copying a makefile from an existing app is the easiest way to get started.
Setup a button callback handler. A button press in Tock is treated as an interrupt, and in an application this translates to a function being called, much like in any other event-driven system. To listen for button presses, we first need to define a callback function, then tell the kernel that the callback exists.
```
#include <stdio.h>
#include <button.h>

// Callback for button presses.
//   btn_num: The index of the button associated with the callback
//   val:     1 if pressed, 0 if depressed
static void button_callback(int btn_num,
                            int val,
                            int arg2 __attribute__ ((unused)),
                            void *user_data __attribute__ ((unused)) ) {
}

int main(void) {
  button_subscribe(button_callback, NULL);

  return 0;
}
```
All callbacks from the kernel are passed four arguments, and the meaning of the four arguments depends on the driver. The first three are integers, and can be used to represent buffer lengths, pin numbers, button numbers, and other simple data. The fourth argument is a pointer to user defined object. This pointer is set in the subscribe call (in this example it is set to NULL), and returned when the callback fires.

Enable the button interrupts. By default, the interrupts for the buttons are not enabled. To enable them, we make a syscall. Buttons, like other drivers in Tock, follow the convention that applications can ask the kernel how many there are. This is done by calling button_count().

#include <stdio.h>
#include <button.h>

// Callback for button presses.
//   btn_num: The index of the button associated with the callback
//   val:     1 if pressed, 0 if depressed
static void button_callback(int btn_num,
                            int val,
                            int arg2 __attribute__ ((unused)),
                            void *user_data __attribute__ ((unused)) ) {
}

int main(void) {
  button_subscribe(button_callback, NULL);

  // Ensure there is a button to use.
  int count = button_count();
  if (count < 1) {
    // There are no buttons on this platform.
    printf("Error! No buttons on this platform.");
  } else {
    // Enable an interrupt on the first button.
    button_enable_interrupt(0);
  }

  // Can just return here. The application will continue to execute.
  return 0;
}

The button count is checked, and the app only continues if there exists at least one button. To enable the button interrupt, button_enable_interrupt() is called with the index of the button to use. In this example we just use the first button.

Call printf() on button press. To print a message, we call printf() in the callback.

#include <stdio.h>
#include <button.h>

// Callback for button presses.
//   btn_num: The index of the button associated with the callback
//   val:     1 if pressed, 0 if depressed
static void button_callback(int btn_num,
                            int val,
                            int arg2 __attribute__ ((unused)),
                            void *user_data __attribute__ ((unused)) ) {
  // Only print on the down press.
  if (val == 1) {
    printf("Hello!\n");
  }
}

int main(void) {
  button_subscribe(button_callback, NULL);

  // Ensure there is a button to use.
  int count = button_count();
  if (count < 1) {
    // There are no buttons on this platform.
    printf("Error! No buttons on this platform.\n");
  } else {
    // Enable an interrupt on the first button.
    button_enable_interrupt(0);
  }

  // Can just return here. The application will continue to execute.
  return 0;
}

Run the application. To try this tutorial application, you can find it in the tutorials app folder. See the first tutorial for details on how to compile and install a C application.

Once installed, when you press the button, you should see "Hello!" printed to the terminal!

The Tock Book

Say "Hello!" On Every Button Press